iwlwifi: Fix mode changes (ad-hoc <--> managed)

This fix allows to move between modes (ad-hoc to managed, and vice versa).
Since mode changes can only be done while driver is down, check for ibss
support can only be made when the channel is set (afte the driver goes up).

Signed-off-by: Assaf Krauss <assaf.krauss@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>